Mendelian Genetics Essay, Research Paper
Gregor Mendel was an Augustinian nineteenth century monk who, due to a series of momentous experiments, is now widely regarded as the forefather of genetics. Mendel studied the inheritance of seven contrasting characteristics of the species Pisum sativum, more commonly known as the garden pea. Each of the variables that Mendel experimented with were discontinuous;
there were no intermediate forms. For example, one of the variables was length of stem, which was always either tall or short. From his experiments Mendel was able to draw solid conclusions about the inheritance of characteristics in organisms. With the advancements in genetics since his time we are now able to explain Mendel’s principles in terms of chromosomes and genes. Understanding of these exact terms did not exist in Mendel’s lifetime. However, Mendel’s principles still form the basis of modern day genetics. In his first series of experiments Mendel allowed Pisum to self-fertilise for several generations, so that he knew that these pea plants were purebred. He then cross-fertilised plants which were purebred for contrasting characteristics. For example, he crossbred pure-bred dwarf Pissum with pure-bred tall Pissum. He carried out reciprocal crosses. Even though these plants obviously showed many characteristics he only looked at one characteristic at a time. In collecting the results of his experiments, Mendel recorded the numbers of individuals in each class in the progeny, this established the ratios of the contrasting characters of many subsequent generations. In the F1 generation all the plants were tall. Mendel then left the F1 generation plants to self-fertilise. In the F2 generation there were both tall and dwarf plants in an approximate ratio of 3:1. The same ratio was found in the F3, F4, and F5 etc. generations. Mendel realised that because the ‘dwarf’ characteristic had disappeared in the F1 and had then reappeared in the F2, the controlling factor for ‘dwarf’ had remained intact and undiluted from one generation to another. It is never expressed, however, in the presence of a factor for ‘tall’. He understood that there must be two independent factors for ‘dwarf’ and ‘tall’. Mendel comprehended that the 3:1 ratio was the product of the binomial expression derived from randomly combining two pairs of unlike elements. We now know Mendel’s ‘factors’ to be genes found on homologous pairs of chromosomes in the nucleus of the cell. There are two or more forms of each gene known as alleles. In Mendel’s experiments the allele in pea plants for ‘tallness’ was dominant and the allele for ‘dwarfness’ was recessive. A pure breeding ‘tall’ plant is homozygous for the ‘tall’ allele and a pure breeding ‘dwarf’ plant is homozygous for the ‘dwarf’ allele. This means that when cross-pollinated the ‘tall’ parent can only pass on gametes containing ‘tall’ alleles and the ‘dwarf’ parent can only pass on gametes containing the ‘dwarf’ allele. As one allele from each gamete combines to form the gene at fertilisation, when a ‘tall’ parent and a ‘dwarf’ parent are crossed, all the offspring must have one ‘tall’ allele and one ‘dwarf’ allele on the gene that codes for length of stem. However, as the ‘tall’ allele is dominant, only this allele was expressed and therefore was shown to be present in all of the F1 generations. Still, when the F1 generation was left to self-fertilise there was a ‘dwarf’ allele present on the genes of all of the plants. There is a fifty percent chance of each allele being in a gamete, so half of the gametes of the F1 generation contained the ‘dwarf’ allele. Therefore if a ‘dwarf’ allele containing gamete and another ‘drawf’ allele containing gamete were to combine the offspring would be homozygous recessive. If a recessive and dominant allele were to combine the offspring in the F2 generations would be identical to their parents still carrying the ‘dwarf’ gene but only expressing the ‘tall’ gene as it is dominant. If A were to stand for the ‘tall’ allele and a for the ‘dwarf allele, if the F1 generation (all being Aa) are allowed to self-fertilise then the offspring would be AA, Aa, aA and aa in a genotypic ratio of 1:2:1 giving the phenotypic ratio of 3:1, which Mendel observed. This can be summed up in Mendel’s first law, which states that ‘The characters of an organism are controlled by pairs of alleles which separate in equal numbers into different gametes as a result of meiosis.’ Mendel also studied the simultaneous inheritance of two characteristics. In one experiment he traced the inheritance of seed colout and texture in Pisum. First he crossed a pure-breeding variety having roundd and yellow seeds with another pure-breeding variety having wrinkled and green seeds. All the F! generation had round, yellow seeds, thus showing these to be the dominant traits. When self-pollinated, the plants which grew from the F1 seeds were round and yellow, round and green, wrinkled and yellow and wrinkled and green in a ratio of 9:3:3:1 respectively. This is a dihybrid ratio. Mendel thus established that dissimilar pairs of factors that combined in a hybrid could separate from one another and come together in all possible combinations in subsequent generations. Mendel did not express his discovery as a law. However, with the information that we now have , his discovery can be stated as his second Law which states that: ‘Two or more pairs of alleles segregate independently of each other as a result of meiosis, provided the genes concerned are not linked by being on the same chromosome.’ The behaviour of chromosomes in meiosis explains how independent segregation occurs. The alleles which determine the two pairs of contrasting characteristics are located on different pairs of homologous autosomes. Because the chromosomes of one pair separate independently of the other pair, the alleles segregate independently. At anaphase I in meiosis the pairs of homologous chromosomes pass to opposite poles of the cell. At Anaphase II the centromeres of the chromosomes break in two and the cromatids are pulled, centromere first, towards opposite poles of the cell, thus becoming four separate sister chromatids. So each of the chromatids separate randomly and independently, thus explaining Mendel’s second law. Drosophilla have been widely used in genetics research. Drosophila was introduced into genetics research by an American biologist, Thomas Hunt Morgan who established the chromosome theory of heredity-the theory that Mendel’s factors are actually the linear series of genes on a chromosome. Morgan’s work established the truth of Mendel’s interpretation of his experiments. Using Drosophila Morgan went on to determine sex linkage, involving the characters that are controlled by genes on sex-determining chromosomes, crossing over, which is the exchange of genes between chromosomes as a result of chiasmata formed during meiosis. He also discovered chromosome maps which show the relative positions of many genes on the four chromosomes of Drosophila. Gregor Mendel, who remained an undiscovered genius for his lifetime prepared the foundations on which modern day genetic are built. His ratios can now be explained in terms of chromosomes and the biochemical processes which take place within cells. Today there are many dire warnings about genetics; dead men are becoming fathers through their frozen sperm, little girls are infused with modified viruses whose infectious qualities have been replaced with healthy genes that the girls lacked at birth and the whole encyclopaedia of the human genome is read, one piece of DNA after another, in perfect sequence, telling us with dreadful accuracy what it means to be normal. On the other hand research is being carried out into genetic diseases, which could save the lives of millions of humans that are alive today and those which have yet to be born. With the discoveries of genetics, which are being made in this rapidly advancing field, comes knowledge along with its associates power and danger. None of these amazing things might be happening today if a monk in a Moravian monastery had not had a particular joy for growing peas in a greenhouse.